The factors giving rise to room echoes (reverberation) and network echoes in telephony and similar voice communications, as well as their effect on speech intelligibility, are well explored in the literature (see, for example, U.S Pat. No. 3,585,311 and Re. 28,919 and, also, the article entitled "Seeking The Ideal In Hands-Free Telephony," Bell Laboratories Record, Volume 52, No. 10, November 1974, page 318 et seq., authored by the present applicants).
In brief, room echoes occur because of room acoustics which result in replicas of speech signals delayed from a few milliseconds to several seconds. Some of these echo signals add to the original speech signals while some of the relatively long ones occur following the termination of the original signals. Because the echoes following the termination of the original signals are not masked by the original signals, they are particularly objectionable. Such unmasked room echoes have been termed "reverberant tails".
Network echoes, on the other hand, occur because of the network and are delayed replicas of the talker's speech as heard by the talker. Such echoes occur because slight imperfections in a network cause some of the talker's speech to be fed back to the talker. These echoes are not added to the original speech other than the fact that they are heard by the talker when speaking.
Previously mentioned U.S. Pat. No. 3,585,311 discloses a speech processing system which uses fixed-level center clipping of a speech signal to reduce network echoes and reverberant tail portions of room echoes. Although this system works satisfactorily in controlled environments and systems, it has been found to be not only inadequate but sometimes troublesome when used in the average telephone application. In particular, it has been found that fixed-level center clipping cannot accommodate: a wide range of network echo conditions produced by variations between networks; a wide range of room echo conditions produced by uncontrolled room conditions; and wide ranges in both network and room echoes produced by uncontrolled talkers.
The invention in the aforementioned U.S. Pat. No. Re. 28,919 provides improved network echo control in the presence of both talker level variability and network variability. In accordance with that invention, a plurality of control signals are derived from voice signals arriving at either end of the network. These control signals are then utilized to adjust the clipping levels in a center clipper located in the outgoing path at that end of the network. In particular, these clipping levels are adjusted so that all of the incoming voice signals which appear on the outgoing path are in effect blocked, whereby network echoes are not heard by the talker at the other end of the network.
Although this configuration provides self-regulation with respect to network echoes, it does not have the ability to reduce, in a self-regulating manner, reverberant tails produced at either end of the network. At best--and as explained in that patent--a limited amount of reverberant tail control can be achieved by providing a finite minimum amount of clipping in the center clipper. Greater control, however, is often desired.